1. Field of the Disclosure
The present disclosure relates to aluminum alloys, particularly 7000 Series (or 7XXX) aluminum (“Al”) alloys as designated by the Aluminum Association. More particularly to aluminum alloy products useful in making structural members for commercial airplanes that are at most 4 inches in thickness.
2. Description of the Related Art
The industry demands on aluminum alloys have become more and more rigorous with each new series of aircraft manufactured by the aerospace industry. As the size of new jet aircraft get larger, or as current jetliner models grow to accommodate heavier payloads and/or longer flight ranges to improve performance and economy, the demand for weight savings in structural components such as wing components continues to increase.
A traditional aircraft wing structure is shown in FIG. 1 and includes a wing box generally designated by numeral 2. Wing box 2 extends outwardly from the fuselage as the main strength component of the wing and runs generally perpendicular to the plane of FIG. 1. In wing box 2, upper and lower wing skins 4 and 6 are spaced by vertical structural members or spars 12 and 20 extending between or bridging upper and lower wing skins. Wing box 2 also includes ribs which extend generally from one spar to the other. These ribs lie parallel to the plane of FIG. 1 whereas the wing skins and spars run perpendicular to the FIG. 1 plane.
The upper wing cover is typically comprised of a skin 4 and stiffening elements or stringers 8. These stiffening elements can be attached separately by fastening or made integral with the skin to eliminate the need for separate stringers and rivets. During flight, the upper wing structure of a commercial aircraft wing is compressively loaded, calling for alloys with high compressive strength. This requirement has led to the development of alloys with increasingly higher compressive strength while still maintaining a nominal level of fracture toughness. The upper wing structural members of today's large aircraft are typically made from high strength 7XXX series aluminum alloys such as 7150 (U.S. Reissue Pat. No. 34,008), 7449 (U.S. Pat. No. 5,560,789) or 7055 aluminum (U.S. Pat. No. 5,221,377). More recently, U.S. Pat. No. 7,097,719 discloses an improved 7055 aluminum alloy.
However, the development of ultra-high capacity aircraft has led to new design requirements. Due to a larger and heavier wing and high aircraft gross takeoff weights, these aircraft experience high down-bending loads during landing producing high tensile loads in the upper wing structural members. While the tensile strength in the current upper wing alloys is more than adequate to withstand these down-bending loads, their fracture toughness becomes a limiting design criterion on the inboard portions of the upper cover. This has led to a desire for alloys for the upper structural members of ultra-large aircraft having very high fracture toughness more akin to that in lower wing skin alloys such as 2324 (U.S. Pat. No. 4,294,625) even if high strength must be sacrificed to some extent. That is, there has been a shift in the optimum combination of strength and toughness needed to maximize weight savings in the upper wing structural members of an ultra-large aircraft to significantly higher fracture toughness and lower strength.
New welding technologies such as friction stir welding have also opened many new possibilities for both design and alloy products for use in wing spar and rib components for weight reduction and/or cost savings. For maximum performance of a spar, the part of the spar which joins to the upper wing skin would have properties similar to the upper skin, and the part of the spar which connects to the lower wing skin would have properties similar to the lower wing skin. This has led to the use of “built-up” spars, comprising an upper spar cap 14 or 22, a web 18 or 20, and a lower spar cap 16 or 24, joined by fasteners (not shown). This “built-up” design allows optimal alloy products to be used for each component. However, the installation of the many fasteners required increases assembly cost. The fasteners and fastener holes may also be structural weak links and parts may have to be thickened which somewhat reduces the performance benefit of using multiple alloys.
One approach used to overcome the assembly cost associated with a built-up spar is to machine the entire spar from a thick plate, extrusion or forging of one alloy. Sometimes, this machining operation is known as “hogging out” the part. With this design, the need for making web-to-upper spar and web-to-lower spar joints is eliminated. A one piece spar fabricated in this manner is sometimes known as an “integral spar”. An ideal alloy for making integral spars should have the strength characteristics of an upper wing alloy combined with the fracture toughness and other damage tolerance characteristics of the lower wing alloy. Typically, achieving both properties simultaneously is difficult and requires a compromise between the property requirements for the upper skin and for the lower skin. One disadvantage that an integral spar must overcome is that the strength and toughness properties of a thick product used as the starting stock are typically less than those of thinner products typically used in a “built-up” spar even if the integral spar is made of the same alloy and temper. Thus, the compromise in properties and the use of thick products for an integral spar may result in a weight penalty. One thick product alloy which reasonably meets the property requirements of both an upper and lower spar cap and retains good properties even in thick products because of its low quench sensitivity, is alloy 7085 described in U.S. Pat. No. 6,972,110. Another disadvantage of integral spars, regardless of alloy, is the high ratio of buy weight (i.e., material which is purchased) to fly weight (i.e., weight of material flying on the aircraft) known as the “buy-to-fly.” This at least partly diminishes the cost advantages of an integral spar over a built-up spar achieved through reduced assembly cost.
However, new technologies such as friction stir welding make further improvements in both weight and cost a possibility. A multi-component spar joined by friction stir welding or other advanced welding or joining methods combines the advantages of a built-up and integral spar. The use of such methods allows the use of use of products of lesser thickness as well as the use of multiple alloys, product forms and/or tempers which are optimized for each spar component. This expands the alloy product/temper options and improves the material buy-to-fly as in a built-up spar, while retaining a significant portion of the assembly cost advantage of an integral spar.
U.S. Pat. No. 5,865,911 describes a 7000 series alloy envisaged for use as lower wing skin structural members and for wing spar members of ultra-high capacity aircraft. This alloy exhibited improvements in strength, toughness, and fatigue resistance in thin plate form relative to incumbent lower wing alloys such as 2024 and 2324 (U.S. Pat. No. 4,294,625). Similar properties in strength and toughness have been obtained in alloy 7085 (U.S. Pat. No. 6,972,110) in thin plate form as shown in Table 1. Either of these alloys in thin product form would be useful for structural members of a lower wing cover and for the lower spar cap and web of a multi-component spar joined by mechanical fastening or welding. These alloys are also suitable for rib applications in either a built-up or integral design. However, the strength levels achievable in these alloys are typically insufficient for use in upper wing structural members of large commercial aircraft. Higher strength is also beneficial for the upper spar cap, spar web and for ribs provided adequate toughness is maintained.
TABLE 1Properties of Miyasato alloy (U.S. Pat. No. 5,865,911)and 7085 (U.S. Pat. No. 6,972,110) in thin plate form.PropertyDirMiyasato (1)7085 (2)UTS (ksi)L82.182.6LT81.482.2TYS (ksi)L76.278.0LT75.477.2Klc, Kq (ksi√in)L-T47.544.0RTT-L40.735.9Klc, Kq (ksi√in)L-T42.040.5−65 F.T-Lna34.3Kapp (ksi√in)L-T120.8128.7RTT-L94.3104.4Kapp (ksi√in)L-T115.5106.8−65 F.T-L74.779.0Kc (ksi√in)L-T172.9165.7RTT-L123.9129.1Kc (ksi√in)L-T166.4140.1−65 F.T-L79.884.8(1) U.S. Pat. No. 5,865,911: Rolled plate 1.2 inches thick, 86 inches wide(2) 7085, U.S. Pat. No. 6,972,110; Rolled plate 1.5 inches thick, 102 inches wide
Thus, a need exists for ultra-high capacity aircraft for an alloy that has significantly higher toughness than current alloys used in upper wing structural members while still maintaining an acceptable level of strength. Such an alloy would also be valuable for use in the upper spar cap and spar web of a multi-component spar joined by mechanical fastening or welding as well as for wing ribs of a built-up or integral design. While the needs of ultra-high capacity aircraft and wings have been specifically discussed such an alloy may also prove beneficial for use in fuselage applications and on smaller aircraft both in built-up and integral structures. In addition, non-aerospace parts such as armor for military vehicles may also be made from the instant alloy.